Polypropylene of high crystallinity has a number of desirable properties that make it a versatile material among current thermoplastic materials. It exhibits high yield strength and rigidity, a high heat distortion temperature and good surface hardness. It is essentially unaffected by most chemicals at or near ambient temperature and is highly resistant to stress cracking. Good scratch and abrasion resistance result in attractive parts. Its impact strength at temperatures above room temperature is moderately good, but polypropylene becomes increasingly brittle at lower temperatures. The ASTM low temperature brittleness point of polypropylene is approximately 70.degree. F. and the polymer becomes extremely brittle at low temperatures, such as at 32.degree. F.
Polypropylene has been used for the manufacture of shaped articles but some of its inherent deficiencies have unfortunately limited its use in many applications for which it would otherwise be well suited. For example, its low impact strength, i.e. high shock sensitivity, at extremely low temperatures is a disadvantage which has precluded its use for many extruded and molded items and in particular for rigid containers and packaging materials. Solid polypropylene is recognized in the plastic industry as possessing great commercial potential because it is superior in several respects to polyethylene, for example, polyproplylene has a higher melting point, lower density and greater stiffness than does polyethylene, which latter material is considered unsuitable for injection molding of certain items such as luggageware, dishwasher proof kitchenware, battery cases, etc.
Very little published data are available on the lowest temperature that different polypropylene compositions will sustain without fracture in standard tests. Some data are available on impact strengths at 0.degree. and 23.degree. C. While these data show that the impact strength decreases markedly as the temperature is lowered, it does not reveal the critical low temperature brittleness point by standard test or imply whether or not molded items will sustain rough handling at more extreme temperatures, such as at -10.degree. F (-23.degree. C) and below.
Attempts to improve the low temperature impact resistance of polypropylene have been made by addition of rubbery materials such as synthetic rubbers, polyisobutylene or amorphous ethylene/propylene random copolymers to polypropylene. The improvements have been minor, since they have been accompanied by less stiffness, lower stability and surface dullness. Also, sever difficulties are encountered in preparing intimate blends of the above mentioned compounds, the rubber material being present as chunks or discrete particles in the extruded blend.
Polypropylene has also been chemically modified by the addition of blocks of either ethylene homopolymer or ethylenepropylene random copolymers to the polypropylene as a means of improving impact strengths at normal and low temperatures.
However, there is a limit to the improvement in impact strength that can be attained by the formation of such block copolymers, since an increase in the amount of ethylene based block in the composition causes a corresponding decrease in the melt flow of the polymer. The net result is that there is a maximum proportion of block that can be incorporated into the polypropylene in order to maintain a melt flow of the composition that renders it suitable for molding, and this maximum proportion is generally insufficient to increase the impact strengths to the desired ranges.
Various polyblends of such ethylene-propylene block copolymers or ethylene-propylene random copolymers with polyethylene and/or polypropylene have been disclosed in the art as a further means of improving the impact strength especially at low temperatures. Although there has been reported favorable results in attaining the desired improvement in impact strength by these blending techniques, there are many disadvantges thereto which often precludes the successful commercialization of such polyblends. For instance, inventory problems, quality control problems, delivery problems, etc. all become more complicated the more components are incorporated in the composition. Also, difficulties are often encountered in preparing completely homogeneous blends resulting in considerable variations in product quality.
A need therefore still exists in the art for a simple polypropylene composition which retains the high tensile strenth and rigidity which is associated with polypropylene, but which has a high impact resistance at very low temperatures while possessing melt flow in the commercially desired ranges.